Human joints are usually more complex than a single axis. The knee joint is among the most complicated synovial joints in the musculoskeletal system. The kinematic studies of knee allow the computation of force distribution during physical activities (such as walking), evaluating surgical operations such as ligament reconstruction, evaluating the effects of inaccurate positioning of condylar prostheses, evaluating the effect on the knee of the use of foot prosthesis, evaluating diagnostic methods for ligament injuries and studying the injury mechanism in a knee joint.
By performing a combination of rolling and sliding, the knee joint accommodates the small contact area between the femur and the tibia. The anatomical structure of the femoral condyles leads to a complex combination of translations and rotations, which includes components of abduction/adduction, internal/external rotations and flexion/extension.
Some tools are known that allow an evaluation of the knee. Instrumented clinical tests as KT1000 [Bach et al., 1990] have been proposed, but their use is still under debate and their reliability and inter-observer reproducibility are questioned [Forster et al., 1989; Huber et al., 1997]. The Lars Rotational Laxiometer [Beacon et al., 1996; Bleday et al., 1998] seems to demonstrate a satisfactory inter and intra-observer reproducibility, but the measurement is limited to the laxity of the knee along one movement axis. Also, considering the 3D nature of the knee's movement, it is essential to complete this measurement by a more global evaluation, in 3D and in movement.
To measure the rotations, localising sensors (magnetic, optic, ultrasonic . . . ) can be used in order to follow the position and orientation of the femur and the tibia in space. Experiments have been made in order to measure the relative motion between the femur and the tibia using such sensors placed on the skin. However, Macleod and Morris (1987) were the first to study the inevitable relative movement between skin and bone during a movement analysis. This has also been done by Sati et al. (1996) who has reported three general methods which address the problem of relative skin movement: 1) use of intracortical pins to fix rigidly but invasively the sensors to the bones, 2) use of statistical calculations to correct the positions of several sensors and 3) use of attachment systems in order to reduce sensors movement in respect with the underlying bone. Only the third method allows having relatively precise measurements of the bone position and orientation, non-invasively. Because these two factors are essential during routine examinations of the knee, the use of an external attachment system seems to be the best compromise.
Sati et al. (1996) proposed an attachment system for the sensors. This mechanical fixation system attaches the sensors onto the underlying bone non-invasively. Three attachment sites onto the condyles are related with a mechanical bridge, which insure the application of inward clamping pressure. A vertical bar insures the system to accurately reflect the orientation of the femoral long axis. The tibial attachment consists of a long bow-shaped plate strapped at both ends to the proximal and distal ends of the tibia. It has been shown that the system can measure knee kinematics with acceptable precision (Sati et al. 1996). This attachment system however revealed some problems in its use:
The mechanical bridge which relates the attachment sites on the femoral harness is designed to be flexible in order to provide comfort to the subject when performing extension of the knee since biceps femoris tendon and ilio-tibial band approach one another during full extension, and the lateral attachment sits on the biceps femoris muscle which has the effect of pushing the lateral attachment away from the knee (Sati, 1996). However, this causes a displacement of the three femoral attachments, particularly on the lateral side, that produces an antero-posterior force which can lead to harness detachment. Also, the localising sensors motion is then influenced by their location on the attachment system.
Moreover, the mechanical bridge flexibility causes orientation changes in part of the harness during subject full extension, which can result in errors in measurements of the position and orientation of the sensors fixed on the harness. Further, the addition of force exerted on knee structures when performing full extension is similar for all subjects. Although it can be acceptable for many subjects, the force can be unbearable for some. Finally, the adjustment and installation is somewhat long and not precise.
A second version of the harness was produced, with a bridge that is rigid in expansion but flexible in torsion, relating one lateral and two medial supports. No lateral expansion is possible during knee extension because of the bridge's rigidity in expansion, which produces an unbearable pressure on both sides of the knee for most of the subjects and causes errors in measurements.
Due to these disadvantages, there is a need to provide a new harness design in order to improve the precision, the sensibility and the reproducibility of the knee analysis system without affecting the subject's comfort.